The present invention is directed toward apparatus for edging reinforced elastomeric stock, hereinafter referred to as a gum edger. With the exception of modern, cast elastomer technology, which does not require reinforcement, it has been well recognized that conventionally employed elastomeric stocks, e.g., rubber, do not possess the inherent strength either to maintain their integrity during the processing steps necessary to obtain the desired article or ultimately to function as an acceptable product when subjected to normal use. Thus, the elastomers have been reinforced in sheet form, generally referred to as reinforced elastomeric fabric, by the inclusion of embedded fibers, mono or polyfilament, which are considerably less extensible than the elastomer. These filaments, or reinforcing cords, include materials such as cotton, synthetics such as rayon, nylon, aramid, polyamides and polyesters, fiberglass and metallic wire, particularly steel, either single strand or cabled.
Calendering has been the historic way in which to make reinforced elastomeric fabric, especially for tire plies. When the elastomer is calendered the reinforcing cords are oriented parallel to the length of the sheet emanating from the calender. As such, in order for the reinforcing cords to be angularly or perpendicularly inclined with respect to a circumferential reference plane when the reinforced elastomeric fabric is incorporated in a tire, it is necessary to cut it either on the bias, or perpendicular to the longitudinal strands of reinforcement.
A more recently developed apparatus for making reinforced elastomeric fabric involves the manufacture of a relatively narrow reinforced ribbon which is cut into strips of predetermined length which strips are subsequently joined and may be stitched together to form the fabric in desired widths wherein the strands of reinforcement are oriented angularly or perpendicularly to the length of the fabric. Suitable apparatus for making fabric in this manner is described in U.S. Pat. No. 3,803,965 and U.S. Ser. No. 676,903, owned by our comon assignee, The Steelastic Company.
Irrespective of the apparatus and method employed, i.e., calendering or assembly of strips of reinforced ribbon, when the reinforcing filaments are metallic, it has been recognized that the exposed ends of reinforcement along the cut edges of the reinforced elastomeric fabric cause an adverse effect upon the products, most particularly radial or bias tires, within which they are incorporated. Metallic reinforcing generally employed is steel wire either monofilament or cabled and tire manufacturers have long striven to obtain good adhesion between the elastomer and embedded metallic reinforcement by incorporating certain rubber soluble cobalt containing salts within the elastomer. Although the wire is plated or coated with brass to resist rusting or an adhesive for enhancement of adhesion, where the wire has been severed, an exposed surface is presented which immediately begins to oxidize upon contact with the atmosphere. While adhesion between chemically clean steel and some elastomers may be acceptable, such oxidation is to be avoided inasmuch as the elastomeric material in which the wire is embedded does not adhere to the wire as it becomes oxidized.
In the manufacture of steel belted bias and radial tires, one or more circumferentially oriented belts are located beneath the tread stock to maintain the integrity and shape of the tire during inflation and subsequent load. The steel reinforcement in these belts is commonly disposed at an angle from the length of the belt, and subsequently with respect to a plane perpendicular to the rotational axis of the tire, and thus, when the belt is constructed, all of the severed ends of steel reinforcement are exposed along both sides of the belt. In addition to making such belts difficult to handle by the worker, oxidation of these exposed ends before the belt can be incorporated in a tire, gives rise to subsequent belt edge separation in the tire.
Belt edge separation is a condition which presently accounts for not only a majority of failure of radial passenger, truck and off-the-road tires but also, for the decrease in tires acceptable for retreading. During use of the tire for distances often exceeding 50,000 to 100,000 miles (80,500 to 161,000 km), these once exposed ends of reinforcement, now surrounded by underlying carcass plies and overlying tread, fail to adhere to the elastomer. With constant flexing and extension of the elastomeric material as well as the steel reinforcement, attendant use of the tire, the edges of the belt eventually break loose from the carcass plies in the region of the shoulder of the tire. Although the condition once started is not curable, if it is not detected or if it be ignored and the tire is not replaced, failure of the tire results by either partial or total separation of the tread from the body of the tire during high speed continued operation.
The latter result is highly intolerable on the highway and has been experienced in this country an appreciable extent on passenger car tires, as well as in other countries where upper speed limits are not posted it is not uncommon for speeds of 100 mph (161 kmph) to be driven for several hours, precipitating such failure with a high probability of disasterous results. While such use of tires may not be expected in this country, that fact alone does not obviate the need to eliminate belt edge separation in steel belted radial tires.
Oxidation of the exposed ends of the steel wire reinforcing may be minimized by incorporating the fabric into the tire soon after it has been made or, by carefully controlling the environment surrounding the fabric prior to its intended use. However, such efforts are rarely practical and the fabric is often stored for a time of from several days to perhaps several months in a warehouse prior to its use and it is during this period that oxidation of the exposed wire reinforcement occurs.
One method developed and employed to eliminate oxidation of the exposed wires is to calender a sheet of a suitable elastomer from which strips may be cut, to be applied to the exposed edges of the reinforced elastomeric fabric, e.g., carcass ply or tread belts. Such strips, referred to as cushion gum, have been relatively wide, e.g., 3.75-5.0 cm wide and approximately 0.038 cm thick, are applied to one side of the edge of the fabric, subsequently folded over onto the other side and are then stitched thereto. However, when the cushion gum is folded over the edge of the fabric, alignment as well as handling is difficult due to the inherent tack of both surfaces, fabric and gum, and inevitably results in an air pocket at the edge where, first, the desired adhesion between steel and elastomer is not obtained and, secondly, wherein oxidation is neither abated nor precluded.
Adhesion-enhancing additive materials are most feasibly employed in cushion gum which is to be applied immediately after the wire shearing operation which provides a chemically clean steel surface for a short period of time. Although this cushion gum is, however, often applied to the belts at the time of tire construction, after the fabric may have been stored for a time sufficient for oxidation of the exposed wire ends to have occurred, failure of the belts in tires so constructed has been found, nevertheless, to be significantly reduced. However, the treatment is not without its faults, which are increased costs in both labor and materials; increased thickness of the ply at the edges; and, the presence of air pockets in the area of the gummed edges which permit oxidation. Moreover, work with the application of various elastomers has demonstrated that the more suitable compounds cannot be calendered into thin strips for subsequent gum edging. Despite many attempts by the industry, gum edging remains a costly, time-consuming process and the disparity between these facts and the benefits gained by the employment of the process remains great enough to limit gum edging to only steel reinforced elastomeric belts for radial truck tires. Considering the benefits in terms of longer tire life and safety, it becomes highly desirable to employ the process of gum edging for all tire manufacture, commerical, e.g., truck tire and off-the-road, and passenger.
Another problem inherent in tires is their adverse effect to internal discontinuities, such as regions where hinge points exist. Hinge points frequently coincide with abrupt endings of the fabric, e.g., endings of the ply turn-ups, endings of break and shock plies, and of belt edges. The recent trend toward reducing tire plies, i.e., 2-ply and 1-ply (radial) passenger car tires in lieu of 4-ply, and 4-ply, 6-ply and 1-ply (radial) in lieu of 12-ply for many of the truck tire sizes, has been greatly facilitated by the use of cord reinforcement of greater diameter, e.g., 1260/3 rather than 840/2, with a consequent increase in the gauge of the elastomeric fabric which, in turn, has contributed significantly to the development of hinge points.
The problem is particularly apparent in the construction of a typical truck tire wherein several plies are cut to a common width and then offset to provide a step-off. When the plies are turned up about the bead region, one side has an exposed step-off and the other side has a buried step-off. Ply endings in the turn-up region not only lead to the possibility of air entrapment, but also, ply distortion in the case of buried turn-ups and an abrupt change in bending stiffness. The overlying ply must make a sharp bend over the buried and exposed step-offs creating a hinge point and tending to trap air. While gum strips may be applied to cover the terminal regions of the plies, and to provide elastomer to flow, the disadvantages are increased cost, a tendency to entrap air and the creation of another edge.
Another ply construction very sensitive to the abrupt ending of the fabric is the buried shock ply--a partial width ply placed within the body of the tire, extending from mid-sidewall to mid-sidewall and lying between the second and third plies. The ends of such plies create a possibility for distortion, air entrapment and hinge points. The application of a gum strip may provide sufficient elastomer to flow for the elimination of some distortion but, in and of itself, provides another discontinuity.
Again, gum edging could be the solution to the elimination of abrupt termination of the plies with a suitable apparatus and method for the application of an elastomer in a configuration and quantity which would facilitate a transition substantially free from distortion between ply endings and overlying plies.